%I #5 Aug 05 2019 07:36:41

%S 0,1,2,4,5,6,8,16,17,21,24,32,34,38,40,56,64,65,66,68,69,70,72,80,81,

%T 85,88,96,98,102,104,120,128,256,257,261,273,277,321,325,337,341,384,

%U 512,514,518,546,550,578,582,610,614,640,896,1024,1025,1026,1028

%N BII-numbers of set-systems (without {}) closed under intersection.

%C A binary index of n is any position of a 1 in its reversed binary expansion. The binary indices of n are row n of A048793. We define the set-system with BII-number n to be obtained by taking the binary indices of each binary index of n. Every finite set of finite nonempty sets has a different BII-number. For example, 18 has reversed binary expansion (0,1,0,0,1), and since the binary indices of 2 and 5 are {2} and {1,3} respectively, the BII-number of {{2},{1,3}} is 18. Elements of a set-system are sometimes called edges.

%e The sequence of all set-systems closed under intersection together with their BII-numbers begins:

%e 0: {}

%e 1: {{1}}

%e 2: {{2}}

%e 4: {{1,2}}

%e 5: {{1},{1,2}}

%e 6: {{2},{1,2}}

%e 8: {{3}}

%e 16: {{1,3}}

%e 17: {{1},{1,3}}

%e 21: {{1},{1,2},{1,3}}

%e 24: {{3},{1,3}}

%e 32: {{2,3}}

%e 34: {{2},{2,3}}

%e 38: {{2},{1,2},{2,3}}

%e 40: {{3},{2,3}}

%e 56: {{3},{1,3},{2,3}}

%e 64: {{1,2,3}}

%e 65: {{1},{1,2,3}}

%e 66: {{2},{1,2,3}}

%e 68: {{1,2},{1,2,3}}

%t bpe[n_]:=Join@@Position[Reverse[IntegerDigits[n,2]],1];

%t Select[Range[0,100],SubsetQ[bpe/@bpe[#],Intersection@@@Tuples[bpe/@bpe[#],2]]&]

%Y The case with union instead of intersection is A326875.

%Y The case closed under union and intersection is A326913.

%Y Set-systems closed under intersection and containing the vertex set are A326903.

%Y Set-systems closed under intersection are A326901, with unlabeled version A326904.

%Y Cf. A006058, A102895, A102898, A326866, A326876, A326878, A326882, A326900, A326902.

%K nonn

%O 1,3

%A _Gus Wiseman_, Aug 04 2019